The present invention relates to a single lens having a refractive index distribution in its radial directions, and especially to the single lens, the chromatic aberration of which is corrected.
Conventionally, the single lens having a homogeneous refractive index is used for a single use camera in which a film is accommodated, and the case of which is disassembled so that the exposed film is taken out after exposure. For the lens having this homogeneous refractive index, a plastic molded lens is used which is made by a process of injection molding to reduce the cost of the lens. Further, acrylic resin material, in which the refractive index of a d, line is approximately 1.49 and an Abbe number is approximately 57, is used for this lens in many cases.
Conventionally, a photographic lens which is a single lens and which has a refractive index distribution has been studied, and the following research papers are disclosed: for example, a study by L. G. Atkinson and et al. ("Applied Optics" Vol. 21 No. 6 p. 993-998), and a study by E. W. Marchand ("Applied Optics" Vol. 19 No. 7 p. 1044-1051).
Further, concerning a lens for an optical disc, which is a single lens and has a refractive index distribution, various patent applications, such as Japanese Patent Publication Open to Public Inspection Nos. 163312/1986, 116613/1989, and the like, have been disclosed and are widely known.
In the foregoing conventional engineering, initially, when the lens having the homogeneous refractive index was used, axial chromatic aberration and transverse chromatic aberration remained in the single lens. These residual chromatic aberrations can not be removed even when an aspherical surface is used for the lens, which is different from other aberrations in monochrome. Further, because parameters of the photographic lens for a camera which is of the single lens type having a homogeneous refractive index are limited in lens design work, it is difficult to balance the spherical aberration and the curvature of the field.
On the other hand, in the photographic lens for a camera, the lens of which is the single lens having a refractive index and was studied by L. G. Atkinson and et al, and E. W. Marchand, lens specifications in which the lens has a relatively smaller F number, that is, an F number of approximately 2 to 6.3, are studied, and a problem remains in which, mainly, astigmatism can not be corrected in the lens.
Further, inventions which have been disclosed in Japanese Patent Publication Open to Public Inspection Nos. 101415/1990, and 176905/1986 are limited to a lens, which has a refractive index distribution in the direction of an optical axis, which is called an axial distribution, and as clearly understandable from the specifications, a radial distribution is basically more effective for aberration correction than the axial distribution.
Further, in many patents with respect to a lens for an optical disc, which is a single lens and has a refractive index distribution, it is not intrinsic characteristic of the lens to correct the chromatic aberration, and an angle of view in the lens specification is approximately satisfied by .omega.=1.degree. , which are basically different from the present invention.